- Email: [email protected]
Deoxidation of Ti–Al intermetallics via hydrogen treatment
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Deoxidation of TieAl intermetallics via hydrogen treatment Yanqing Su*, Xinwang Liu, Liangshun Luo, Long Zhao, Jingjie Guo, Hengzhi Fu National Key Laboratory of Science and Technology for Precision Heat Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China
article info
abstract
Article history:
Alloys of Tie47Al were deoxidized via a simple method called hydrogen treatment (HT),
Received 23 March 2010
which involves deoxidation with hydrogen in a melting process. Because of the increase in
Received in revised form
the partial pressure of hydrogen and the melting duration, the oxygen content of the alloys
25 May 2010
greatly decreased after HT. Activated hydrogen atoms dissociated at high temperatures,
Accepted 26 May 2010
and the hydrogen molecules in the melting chamber seemed to affect the deoxidation
Available online 14 July 2010
reactions, which are represented by the following equations: O þ 2H ¼ H2O and O þ H2 ¼ H2O. Based on a comparison of the changes in Gibbs free energy, the hydrogen
Keywords: Hydrogen treatment
atoms were found to play a major role in deoxidation. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Intermetallics Deoxidation Free energy
1.
Introduction
Titaniumealuminum (TieAl)-based intermetallics are candidate materials for replacing nickel-based superalloys in some gas turbine engine applications, such as low-pressure or power turbine blades [1]. The critical barrier for commercial applications of TieAl alloys is their low ductility and poor fracture resistance at room temperatures [2]. A detailed study was carried out to study the deformation behaviors of TieAl-based alloys containing different levels of oxygen at room temperature, which revealed that oxygen had negative affection on the room ductility of TieAl alloys [3,4]. The extensive glide of ordinary 1/2<110> dislocations occurs in the case of high-purity alloys and these alloys exhibit some tensile ductility. However, the activity of these dislocations is very much reduced with increasing oxygen content, which may be due to pinning along the dislocation lines by oxygen [3]. And TieAl alloys with high oxygen
content deform by superdislocations and are totally brittle at room temperature. The ductility of TieAl alloys becomes higher with decreasing oxygen content. So for better applications of TieAl alloy, it is expected that the oxygen content is as low as possible. TieAl alloys are melted mostly in vacuum melting apparatuses, examples of which include plasma-arc melt furnaces [5]. These alloys easily absorb oxygen during the melting process. Their primary materials also contain some oxygen, thus further increasing the oxygen content. However, oxygen has harmful influences on the mechanical properties of TieAl alloys as described above, so it is necessary to deoxidize them prior to their application [6]. A common method for deoxidation of TieAl alloys is the calciumealuminum (CaeAl) method [7]. Unfortunately, it was found that some Ca remained in the alloys post-deoxidation. Thus, researchers focused their efforts on discovering a deoxidation method that did not leave unwanted elements in the alloy.
* Corresponding author. Tel.: þ86 451 86417395; fax: þ86 451 86415776. E-mail address: [email protected] (Y. Su). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.115
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
In this study, hydrogen treatment (HT) is used to deoxidize TieAl alloys. In the HT process, TieAl alloys are melted in the presence of hydrogen. Deoxidation occurs via a series of reactions of hydrogen and oxygen. In this way, any hydrogen existing in the samples can be easily removed by vacuum annealing prior to their application. Possibly the remnant hydrogen can be utilized as a temporary element to improve the processing of TieAl alloys, including sintering, compacting, machining, and hot working before dehydrogenation [8e10]. This process does not retain other phases or elements. The process can also be completed in the same amount of time that the melting process takes. Thus, HT is more likely to be an efficient method for deoxidation of TieAl alloys than other approaches. The aim of the present work is to validate the effects of the HT method on the deoxidation of Tie47Al alloys (all expressed in terms of at.% in this paper). The deoxidation mechanism of hydrogen is also discussed.
2.
Experimental
TieAl binary alloys were selected for this study. In order to compare the deoxidation effects of HT more clearly, pre-cast ingots containing specific amounts of oxygen were first prepared. To better understand the deoxidation of TieAl alloys by HT, oxygen in the form of TiO2 powder was added to the system prior to melting. The TieAl alloy ingots were prepared using a water-cooled copper crucible in a non-consumable electrode arc furnace under an atmosphere of pure argon. The Ti sponge and high-purity Al (>99.99%) were used for the preparation of TieAl alloys. About 25e30 g of alloy buttons were melted 4e5 times in order to improve chemical homogeneity. The nominal composition of the TieAl alloy is Tie47Al. Deoxidation was performed with hydrogen treatment consisting of a non-consumable tungsten electrode arc melting furnace and a hydrogen analyzer. A schematic diagram of this method is shown in Fig. 1. During deoxidation, TieAl ingots were melted under a gaseous mixture of hydrogen and argon in the furnace. The hydrogen analyzer then detected both the volume fraction of hydrogen and its pressure in the melting chamber, and the partial pressure of hydrogen in the system could be controlled by the hydrogen
9215
analyzer. Different partial pressures of hydrogen and different melting durations were studied for their effects on deoxidation via HT. The temperature of the alloy melt was measured by an infrared thermoscope. The oxygen content was obtained from chemical analysis. Dehydrogenation was performed by vacuum annealing at 1023 K for 2 h. During this process, the pressure of the furnace chamber was kept at 104 Pa. After that, the hydrogen content of the alloys was determined by chemical analysis.
3.
Result and discussion
Fig. 2(a) shows the changes in the oxygen content of the Tie47Al alloys after HT with different partial pressures of hydrogen. Two sets of ingots with different levels of starting oxygen content were conducted by HT, which is marked by A and B respectively. The oxygen content of the second set with high starting oxygen content after HT was divided by 8 for clearer comparison, which is shown by B. The melting time was set at 240 s. During the course of the experiment, the oxygen content of the alloys decreased rapidly with an increase in the partial pressure of hydrogen. When the partial pressure of hydrogen reached 10 kPa, however, the oxygen content remained steady and no longer decreased. Thus, we propose that a hydrogen partial pressure of 10 kPa can remove a significant amount of oxygen via HT. This finding also allows us to conclude that HT is an effective method for deoxidizing TieAl alloys. Changes in the oxygen content of the Tie47Al alloys with respect to increasing melting time are shown in Fig. 2(b). In this set of experiments, hydrogen partial pressures of 10 and 20 kPa were used. The oxygen content in the alloys clearly decreased with an increase in the melting duration. Initially, the oxygen content decreased rapidly until the melting duration reached 360 s, after which the oxygen content no longer decreased. Thus, we conclude that a melting duration of 360 s is sufficient for deoxidation of TieAl alloys via HT. Increasing the partial hydrogen pressure from 10 kPa to 20 kPa resulted in minor effects on the deoxidation process. When the melting duration exceeded 360 s, the deoxidation effects for both pressure conditions were nearly the same. In order to discuss the deoxidation mechanism of HT, it is necessary to first understand the behavior of hydrogen, particularly those at the surface of the arc zone and the molten pool. The temperatures of the arc and melt were high enough for diatomic hydrogen to dissociate into monatomic hydrogen. Some diatomic hydrogen could be excited into ionized hydrogen by electrons emitted from the cathode. The hydrogen atoms or ions moved with the arc and reached the melt surface. Some of the hydrogen that reached the alloy melt surface dissociated into monatomic hydrogen and diffused into the melt. This process could be described by the equation:
½H2 (g) ¼ H (atom) in liquid metal, DG0 (J) ¼ 44,780 þ 3.38T(1) Fig. 1 e Schematic diagram of the hydrogen charging system.
Some hydrogen atoms could also combine to form molecules on the melt surface and escape into the melting
9216
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
Fig. 2 e Changes in the oxygen content of the Tie47Al alloys with variations in (a) partial pressure of hydrogen, and (b) melting duration. The values of B in (a) were modified from their original ones divided by 8.
chamber. With the above reactions, more and more hydrogen could diffuse into the TieAl melt. When the alloy melt was saturated with hydrogen, a dynamic equilibrium between the alloy melt and the melting chamber was achieved. The process through which this dynamic equilibrium is attained is described by Sieverts [11]. There are always activated hydrogen atoms on the melt surface [12], and deoxidation reactions are considered to take place on the melt surface. Deoxidation by hydrogen includes two reactions with both hydrogen atoms and molecules on the melt surface. When the oxygen atoms in the alloy melt move to the melt surface, they can combine with the hydrogen atoms that come from the alloy melt or simply dissociate on the surface and form water molecules. Oxygen can also react with the hydrogen molecules that are near the melt surface to generate water [13]. This water stream then flows with the mixture of argon and hydrogen. It is reported that hydrogen can also remove oxygen from Zr, Nb and Ta metals based on the same reactions [14]. These processes can be expressed in the following equations: O (in melt) þ 2H ¼ H2O, DG0 (J) ¼ 476,778 þ 110.16T
(2)
O (in melt) þ H2 ¼ H2O, DG0 (J) ¼ 24,476 10.96T
(3)
The schematic of the deoxidation reactions is shown in Fig. 3, in which the reactions involved in deoxidation are marked. Fig. 4 shows the dependence of the Gibbs free energy (DG0) on the temperature for the deoxidation by hydrogen. The Gibbs free energy for Eq. (4) had the most negative value, indicating that this reaction was the easiest to initiate. For this study, however, there was limited residual air in the chamber and the reactions of Eq. (4) were very weak. In this study, reaction (4) play a minimal role in deoxidation. Deoxidation was believed to have been achieved mainly by reactions between the hydrogen and the oxygen atoms. The deoxidation processes by H2 and H were both feasible, since the values of DG0 for both were negative. However, the process initiated by H2 was estimated to be much more difficult than the process initiated by H because the values of DG0 for H2 were much higher and closer to zero than the latter. Thus, deoxidation is likely to be controlled by Eq. (2). Most of the oxygen in the system was considered to have been removed by hydrogen atoms. When the temperature increased, the free energy of Eq. (2) increased rapidly
In addition to deoxidation from the melt, there is also another reaction that might also benefit deoxidation. Some residual air containing oxygen might exist in the melting chamber, even though the molecular pump attempted to somewhat evacuate this space. As such, hydrogen could also react with the oxygen near the arc zone or melt surface where the temperature was higher than the ignition point of hydrogen. This process is described by the following equation:
½O2 þ H2 ¼ H2O, DG0 (J) ¼ 247,500 55.86T
(4)
Fig. 3 e Schematic diagram of the deoxidation reactions.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
9217
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (50975060) and the Foundation of State Key Laboratory of Advanced Welding Production Technology of China for their financial support.
references
Fig. 4 e Dependence of the Gibbs free energy on temperature for the deoxidation by hydrogen.
and that of Eq. (3) decreased very slowly. As such, deoxidation became more difficult as the temperature increased. To obtain better deoxidation results, the superheat should be kept as low as possible. After the HT process, the hydrogen which dissolved into the TieAl alloy melt remained in the alloy during the solidification process and formed hydrides, which might induce brittleness in TieAl alloys [15]. This left-over hydrogen could be easily removed by vacuum annealing [16,17]. After dehydrogenation, chemical analysis was performed to determine the left-over hydrogen content, which indicated that the hydrogen was reduced below 50 weight ppm by 2 h of vacuum annealing. This confirmed that hydrogen treatment is a feasible method for deoxidation of TieAl alloys.
4.
Conclusions
Alloys of Tie47Al were subjected to hydrogen treatment to determine the deoxidation effects. The method showed good deoxidation activity on TieAl alloys, as evidenced by the decrease in oxygen content of the alloys when the partial pressure of hydrogen and the melting duration were increased. A partial pressure of 10 kPa and a melting duration of 360 s were the optimized effective parameters for deoxidation of these alloys. The dissociated hydrogen atoms and molecules might both play roles in the deoxidation reactions. Of the two, however, hydrogen atoms were considered to play a bigger role, as its Gibbs free energy was found to be much lower (more negative) than that for the hydrogen molecules.
[1] Saari H, Beddoes J, Seo DY, Zhao L. Development of directionally solidified g-TiAl structures. Intermetallics 2005;13:937e43. [2] Hu D. Effect of boron addition on tensile ductility in lamellar TiAl alloys. Intermetallics 2002;10:851e8. [3] Morris MA. Dislocation mobility, ductility and anomalous strengthening of two-phase TiAl alloys: effects of oxygen and composition. Intermetallics 1996;4:417e26. [4] Wu Y, Hwang SK. Microstructural refinement and improvement of mechanical properties and oxidation resistance in EPM TiAl-based intermetallics with yttrium addition. Acta Mater 2002;50:1479e93. [5] Sugilal G. Experimental study of natural convection in a glass pool inside a cold crucible induction melter. Int J Therm Sci 2008;47:918e25. [6] Noda T. Application of cast gamma TiAl for automobiles. Intermetallics 1998;6:709e13. [7] Okabe TH, Oishi T, Ono K. Deoxidation of titanium aluminide by CaeAl alloy under controlled aluminum activity. Metall Mater Trans B 1992;23:583e90. [8] Sun ZG, Zhou WL, Hou HL. Strengthening of Tie6Ale4V alloys by thermohydrogen processing. Int J Hydrogen Energy 2009;34:1971e6. [9] Murzinova MA, Salishchev GA, Afonichev DD. Formation of nanocrystalline structure in two-phase titanium alloy by combination of thermohydrogen processing with hot working. Int J Hydrogen Energy 2002;27:775e82. [10] Liu HJ, Zhou L, Liu P, Liu QW. Microstructural evolution and hydride precipitation mechanism in the hydrogenated Tie6Ale4V alloy. Int J Hydrogen Energy 2009;34:9596e602. [11] Sieverts A. Absorption of gases by metals. Zeitschrift fur Metallkunde 1929;21:37e46. [12] Su YQ, Wang L, Luo LS, Jiang XH, Guo JJ, Fu HZ. Deoxidation of titanium alloy using hydrogen. Int J Hydrogen Energy 2009; 34:8958e63. [13] Mimura K, Komukai T, Isshiki M. Purification of chromium by hydrogen plasma-arc zone melting. Mater Sci Eng A 2005; 403:11e6. [14] Elanski D, Limb JW, Mimura K, Isshiki M. Impurity removal from Zr, Nb and Ta metals by hydrogen plasma arc melting and thermodynamic estimation of hydride formation. J Alloys Compd 2006;413:251e8. [15] Chu WY, Thompson AW. Effect of microstructure and hydrides on fracture of TiAl. Scripta Metall Mater 1991;25:2133e8. [16] Senkov ON, Froes FH. Thermohydrogen processing of titanium alloys. Int J Hydrogen Energy 1999;24:565e76. [17] Froes FH, Senkov ON, Qazi JI. Hydrogen as a temporary alloying element in titanium alloys: thermohydrogen processing. Int Mater Rev 2004;49:227e45.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Deoxidation of TieAl intermetallics via hydrogen treatment Yanqing Su*, Xinwang Liu, Liangshun Luo, Long Zhao, Jingjie Guo, Hengzhi Fu National Key Laboratory of Science and Technology for Precision Heat Processing of Metals, Harbin Institute of Technology, Harbin 150001, PR China
article info
abstract
Article history:
Alloys of Tie47Al were deoxidized via a simple method called hydrogen treatment (HT),
Received 23 March 2010
which involves deoxidation with hydrogen in a melting process. Because of the increase in
Received in revised form
the partial pressure of hydrogen and the melting duration, the oxygen content of the alloys
25 May 2010
greatly decreased after HT. Activated hydrogen atoms dissociated at high temperatures,
Accepted 26 May 2010
and the hydrogen molecules in the melting chamber seemed to affect the deoxidation
Available online 14 July 2010
reactions, which are represented by the following equations: O þ 2H ¼ H2O and O þ H2 ¼ H2O. Based on a comparison of the changes in Gibbs free energy, the hydrogen
Keywords: Hydrogen treatment
atoms were found to play a major role in deoxidation. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Intermetallics Deoxidation Free energy
1.
Introduction
Titaniumealuminum (TieAl)-based intermetallics are candidate materials for replacing nickel-based superalloys in some gas turbine engine applications, such as low-pressure or power turbine blades [1]. The critical barrier for commercial applications of TieAl alloys is their low ductility and poor fracture resistance at room temperatures [2]. A detailed study was carried out to study the deformation behaviors of TieAl-based alloys containing different levels of oxygen at room temperature, which revealed that oxygen had negative affection on the room ductility of TieAl alloys [3,4]. The extensive glide of ordinary 1/2<110> dislocations occurs in the case of high-purity alloys and these alloys exhibit some tensile ductility. However, the activity of these dislocations is very much reduced with increasing oxygen content, which may be due to pinning along the dislocation lines by oxygen [3]. And TieAl alloys with high oxygen
content deform by superdislocations and are totally brittle at room temperature. The ductility of TieAl alloys becomes higher with decreasing oxygen content. So for better applications of TieAl alloy, it is expected that the oxygen content is as low as possible. TieAl alloys are melted mostly in vacuum melting apparatuses, examples of which include plasma-arc melt furnaces [5]. These alloys easily absorb oxygen during the melting process. Their primary materials also contain some oxygen, thus further increasing the oxygen content. However, oxygen has harmful influences on the mechanical properties of TieAl alloys as described above, so it is necessary to deoxidize them prior to their application [6]. A common method for deoxidation of TieAl alloys is the calciumealuminum (CaeAl) method [7]. Unfortunately, it was found that some Ca remained in the alloys post-deoxidation. Thus, researchers focused their efforts on discovering a deoxidation method that did not leave unwanted elements in the alloy.
* Corresponding author. Tel.: þ86 451 86417395; fax: þ86 451 86415776. E-mail address: [email protected] (Y. Su). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.115
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
In this study, hydrogen treatment (HT) is used to deoxidize TieAl alloys. In the HT process, TieAl alloys are melted in the presence of hydrogen. Deoxidation occurs via a series of reactions of hydrogen and oxygen. In this way, any hydrogen existing in the samples can be easily removed by vacuum annealing prior to their application. Possibly the remnant hydrogen can be utilized as a temporary element to improve the processing of TieAl alloys, including sintering, compacting, machining, and hot working before dehydrogenation [8e10]. This process does not retain other phases or elements. The process can also be completed in the same amount of time that the melting process takes. Thus, HT is more likely to be an efficient method for deoxidation of TieAl alloys than other approaches. The aim of the present work is to validate the effects of the HT method on the deoxidation of Tie47Al alloys (all expressed in terms of at.% in this paper). The deoxidation mechanism of hydrogen is also discussed.
2.
Experimental
TieAl binary alloys were selected for this study. In order to compare the deoxidation effects of HT more clearly, pre-cast ingots containing specific amounts of oxygen were first prepared. To better understand the deoxidation of TieAl alloys by HT, oxygen in the form of TiO2 powder was added to the system prior to melting. The TieAl alloy ingots were prepared using a water-cooled copper crucible in a non-consumable electrode arc furnace under an atmosphere of pure argon. The Ti sponge and high-purity Al (>99.99%) were used for the preparation of TieAl alloys. About 25e30 g of alloy buttons were melted 4e5 times in order to improve chemical homogeneity. The nominal composition of the TieAl alloy is Tie47Al. Deoxidation was performed with hydrogen treatment consisting of a non-consumable tungsten electrode arc melting furnace and a hydrogen analyzer. A schematic diagram of this method is shown in Fig. 1. During deoxidation, TieAl ingots were melted under a gaseous mixture of hydrogen and argon in the furnace. The hydrogen analyzer then detected both the volume fraction of hydrogen and its pressure in the melting chamber, and the partial pressure of hydrogen in the system could be controlled by the hydrogen
9215
analyzer. Different partial pressures of hydrogen and different melting durations were studied for their effects on deoxidation via HT. The temperature of the alloy melt was measured by an infrared thermoscope. The oxygen content was obtained from chemical analysis. Dehydrogenation was performed by vacuum annealing at 1023 K for 2 h. During this process, the pressure of the furnace chamber was kept at 104 Pa. After that, the hydrogen content of the alloys was determined by chemical analysis.
3.
Result and discussion
Fig. 2(a) shows the changes in the oxygen content of the Tie47Al alloys after HT with different partial pressures of hydrogen. Two sets of ingots with different levels of starting oxygen content were conducted by HT, which is marked by A and B respectively. The oxygen content of the second set with high starting oxygen content after HT was divided by 8 for clearer comparison, which is shown by B. The melting time was set at 240 s. During the course of the experiment, the oxygen content of the alloys decreased rapidly with an increase in the partial pressure of hydrogen. When the partial pressure of hydrogen reached 10 kPa, however, the oxygen content remained steady and no longer decreased. Thus, we propose that a hydrogen partial pressure of 10 kPa can remove a significant amount of oxygen via HT. This finding also allows us to conclude that HT is an effective method for deoxidizing TieAl alloys. Changes in the oxygen content of the Tie47Al alloys with respect to increasing melting time are shown in Fig. 2(b). In this set of experiments, hydrogen partial pressures of 10 and 20 kPa were used. The oxygen content in the alloys clearly decreased with an increase in the melting duration. Initially, the oxygen content decreased rapidly until the melting duration reached 360 s, after which the oxygen content no longer decreased. Thus, we conclude that a melting duration of 360 s is sufficient for deoxidation of TieAl alloys via HT. Increasing the partial hydrogen pressure from 10 kPa to 20 kPa resulted in minor effects on the deoxidation process. When the melting duration exceeded 360 s, the deoxidation effects for both pressure conditions were nearly the same. In order to discuss the deoxidation mechanism of HT, it is necessary to first understand the behavior of hydrogen, particularly those at the surface of the arc zone and the molten pool. The temperatures of the arc and melt were high enough for diatomic hydrogen to dissociate into monatomic hydrogen. Some diatomic hydrogen could be excited into ionized hydrogen by electrons emitted from the cathode. The hydrogen atoms or ions moved with the arc and reached the melt surface. Some of the hydrogen that reached the alloy melt surface dissociated into monatomic hydrogen and diffused into the melt. This process could be described by the equation:
½H2 (g) ¼ H (atom) in liquid metal, DG0 (J) ¼ 44,780 þ 3.38T(1) Fig. 1 e Schematic diagram of the hydrogen charging system.
Some hydrogen atoms could also combine to form molecules on the melt surface and escape into the melting
9216
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
Fig. 2 e Changes in the oxygen content of the Tie47Al alloys with variations in (a) partial pressure of hydrogen, and (b) melting duration. The values of B in (a) were modified from their original ones divided by 8.
chamber. With the above reactions, more and more hydrogen could diffuse into the TieAl melt. When the alloy melt was saturated with hydrogen, a dynamic equilibrium between the alloy melt and the melting chamber was achieved. The process through which this dynamic equilibrium is attained is described by Sieverts [11]. There are always activated hydrogen atoms on the melt surface [12], and deoxidation reactions are considered to take place on the melt surface. Deoxidation by hydrogen includes two reactions with both hydrogen atoms and molecules on the melt surface. When the oxygen atoms in the alloy melt move to the melt surface, they can combine with the hydrogen atoms that come from the alloy melt or simply dissociate on the surface and form water molecules. Oxygen can also react with the hydrogen molecules that are near the melt surface to generate water [13]. This water stream then flows with the mixture of argon and hydrogen. It is reported that hydrogen can also remove oxygen from Zr, Nb and Ta metals based on the same reactions [14]. These processes can be expressed in the following equations: O (in melt) þ 2H ¼ H2O, DG0 (J) ¼ 476,778 þ 110.16T
(2)
O (in melt) þ H2 ¼ H2O, DG0 (J) ¼ 24,476 10.96T
(3)
The schematic of the deoxidation reactions is shown in Fig. 3, in which the reactions involved in deoxidation are marked. Fig. 4 shows the dependence of the Gibbs free energy (DG0) on the temperature for the deoxidation by hydrogen. The Gibbs free energy for Eq. (4) had the most negative value, indicating that this reaction was the easiest to initiate. For this study, however, there was limited residual air in the chamber and the reactions of Eq. (4) were very weak. In this study, reaction (4) play a minimal role in deoxidation. Deoxidation was believed to have been achieved mainly by reactions between the hydrogen and the oxygen atoms. The deoxidation processes by H2 and H were both feasible, since the values of DG0 for both were negative. However, the process initiated by H2 was estimated to be much more difficult than the process initiated by H because the values of DG0 for H2 were much higher and closer to zero than the latter. Thus, deoxidation is likely to be controlled by Eq. (2). Most of the oxygen in the system was considered to have been removed by hydrogen atoms. When the temperature increased, the free energy of Eq. (2) increased rapidly
In addition to deoxidation from the melt, there is also another reaction that might also benefit deoxidation. Some residual air containing oxygen might exist in the melting chamber, even though the molecular pump attempted to somewhat evacuate this space. As such, hydrogen could also react with the oxygen near the arc zone or melt surface where the temperature was higher than the ignition point of hydrogen. This process is described by the following equation:
½O2 þ H2 ¼ H2O, DG0 (J) ¼ 247,500 55.86T
(4)
Fig. 3 e Schematic diagram of the deoxidation reactions.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4 e9 2 1 7
9217
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (50975060) and the Foundation of State Key Laboratory of Advanced Welding Production Technology of China for their financial support.
references
Fig. 4 e Dependence of the Gibbs free energy on temperature for the deoxidation by hydrogen.
and that of Eq. (3) decreased very slowly. As such, deoxidation became more difficult as the temperature increased. To obtain better deoxidation results, the superheat should be kept as low as possible. After the HT process, the hydrogen which dissolved into the TieAl alloy melt remained in the alloy during the solidification process and formed hydrides, which might induce brittleness in TieAl alloys [15]. This left-over hydrogen could be easily removed by vacuum annealing [16,17]. After dehydrogenation, chemical analysis was performed to determine the left-over hydrogen content, which indicated that the hydrogen was reduced below 50 weight ppm by 2 h of vacuum annealing. This confirmed that hydrogen treatment is a feasible method for deoxidation of TieAl alloys.
4.
Conclusions
Alloys of Tie47Al were subjected to hydrogen treatment to determine the deoxidation effects. The method showed good deoxidation activity on TieAl alloys, as evidenced by the decrease in oxygen content of the alloys when the partial pressure of hydrogen and the melting duration were increased. A partial pressure of 10 kPa and a melting duration of 360 s were the optimized effective parameters for deoxidation of these alloys. The dissociated hydrogen atoms and molecules might both play roles in the deoxidation reactions. Of the two, however, hydrogen atoms were considered to play a bigger role, as its Gibbs free energy was found to be much lower (more negative) than that for the hydrogen molecules.
[1] Saari H, Beddoes J, Seo DY, Zhao L. Development of directionally solidified g-TiAl structures. Intermetallics 2005;13:937e43. [2] Hu D. Effect of boron addition on tensile ductility in lamellar TiAl alloys. Intermetallics 2002;10:851e8. [3] Morris MA. Dislocation mobility, ductility and anomalous strengthening of two-phase TiAl alloys: effects of oxygen and composition. Intermetallics 1996;4:417e26. [4] Wu Y, Hwang SK. Microstructural refinement and improvement of mechanical properties and oxidation resistance in EPM TiAl-based intermetallics with yttrium addition. Acta Mater 2002;50:1479e93. [5] Sugilal G. Experimental study of natural convection in a glass pool inside a cold crucible induction melter. Int J Therm Sci 2008;47:918e25. [6] Noda T. Application of cast gamma TiAl for automobiles. Intermetallics 1998;6:709e13. [7] Okabe TH, Oishi T, Ono K. Deoxidation of titanium aluminide by CaeAl alloy under controlled aluminum activity. Metall Mater Trans B 1992;23:583e90. [8] Sun ZG, Zhou WL, Hou HL. Strengthening of Tie6Ale4V alloys by thermohydrogen processing. Int J Hydrogen Energy 2009;34:1971e6. [9] Murzinova MA, Salishchev GA, Afonichev DD. Formation of nanocrystalline structure in two-phase titanium alloy by combination of thermohydrogen processing with hot working. Int J Hydrogen Energy 2002;27:775e82. [10] Liu HJ, Zhou L, Liu P, Liu QW. Microstructural evolution and hydride precipitation mechanism in the hydrogenated Tie6Ale4V alloy. Int J Hydrogen Energy 2009;34:9596e602. [11] Sieverts A. Absorption of gases by metals. Zeitschrift fur Metallkunde 1929;21:37e46. [12] Su YQ, Wang L, Luo LS, Jiang XH, Guo JJ, Fu HZ. Deoxidation of titanium alloy using hydrogen. Int J Hydrogen Energy 2009; 34:8958e63. [13] Mimura K, Komukai T, Isshiki M. Purification of chromium by hydrogen plasma-arc zone melting. Mater Sci Eng A 2005; 403:11e6. [14] Elanski D, Limb JW, Mimura K, Isshiki M. Impurity removal from Zr, Nb and Ta metals by hydrogen plasma arc melting and thermodynamic estimation of hydride formation. J Alloys Compd 2006;413:251e8. [15] Chu WY, Thompson AW. Effect of microstructure and hydrides on fracture of TiAl. Scripta Metall Mater 1991;25:2133e8. [16] Senkov ON, Froes FH. Thermohydrogen processing of titanium alloys. Int J Hydrogen Energy 1999;24:565e76. [17] Froes FH, Senkov ON, Qazi JI. Hydrogen as a temporary alloying element in titanium alloys: thermohydrogen processing. Int Mater Rev 2004;49:227e45.